Referring to FIG. 1, a typical permanent magnet (PM) alternator or motor has a rotor 102 supporting permanent magnets 104 and mounted on a rotatable shaft 108. A stator 110 has a plurality of windings 112 between a plurality of teeth 114 mounted to a back iron 116. (For ease of illustration, the adjacent elements of windings 112 in FIG. 1 are shown unconnected.) When operated in an alternator mode, an external torque source forces rotation of the shaft, and the interaction of the magnets and the windings causes a magnetic flux to loop the windings in the slots. As magnetic flux varies due to rotor rotation, voltage is generated in windings 112, which results in an output current when a load is connected to the output of the machine. When operated in a motor mode, voltage from an external source (not shown) is applied to windings 112, and the resulting current induces magnetic flux in the stator and rotor which, when appropriately controlled, causes the rotor to rotate to produce torque. PM machines can have an “inside rotor” configuration as shown in FIG. 1 or an “outside rotor” configuration (not shown).
The output voltage and frequency of prior art PM alternators is determined by rotor speed, which present challenges where rotor speed cannot be independently controlled. It would therefore be desirable to improve the controllability of electric machines, generally, and in particular PM machines.